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INVESTIGATION ON NON-LINEAR GRADING MATERIAL AS STRESS

CONTROL ON MEDIUM VOLTAGE CABLE TERMINATION

SITI NUR AISHAH BINTI MAHD RAWE

A project report submitted in partial

fulfillment of the requirements for the award of the

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering,

Universiti Tun Hussein Onn Malaysia

JANUARY 2014

v

ABSTRACT

Power cable terminations are widely used in power system networks for a last few

decades. A proper design of cable termination is essential in reducing the electric

field distribution around the end of high voltage cable. At the termination of cable,

the electric field was usually the highest, for the insulation layer was cut in this place.

And with the high electric field at the cutting point, there were surely partial

discharge occurs. In order to overcome this issue, some sorts of method were

proposed. Stress control geometry method and non-linear properties of Zinc Oxide

Microvaristor method have been used in this research. The aim was to find an

optimal solution for the electric field distribution. An 11kV power cable has been

modelled for this study by using Finite Elements Method (FEM) computational

solution. The model has been used to simulate the electric field distribution in the

cable termination. The structure of terminations and the critical parts in this design

were also defined through literature. A non-linear grading has been applied to several

design of layer insulation. The proposed model was used to covers the point that

generate high fields at screen cable insulation. Material properties play a key role in

suppresses electric field spread throughout termination area. Based on the switching

point of about 1kV/cm the material changes its conductivity significantly. In this way

it prevents the occurrence of electric fields which are above this switching point.

Different thicknesses also were introduced to get a better design and economically to

produce in cost advantages. This research successfully examines the effectiveness

for both methods in controlling field stress at cable screen termination.

vi

ABSTRAK

Penamatan kabel kuasa digunakan secara meluas dalam rangkaian sistem kuasa

selama beberapa dekad yang lalu. Apabila lapisan penebat penamatan kabel

dipotong, keadaan ini akan menghasilkan medan elektrik yang tinggi. Medan elektrik

yang tinggi pada pemotongan lapisan penebat kabel ini, akan terhasilnya pelepasan

medan elektrik . Untuk mengatasi isu ini, beberapa jenis kaedah telah dicadangkan.

Kawalan tekanan kaedah geometri dan ciri-ciri yang tidak linear iaitu Zink Oksida

‘Microvaristor’ telah digunakan dalam kajian ini. Sasaran utama kajian ini adalah

untuk mencari penyelesaian optimum bagi taburan medan elektrik. Kabel kuasa

11kV telah dimodelkan untuk kajian ini dengan menggunakan „Finite Element

Method’ (FEM) penyelesaian pengiraan. Model ini telah digunakan untuk simulasi

taburan medan elektrik penamatan kabel. Struktur penamatan dan bahagian-bahagian

penting dalam reka bentuk ini juga ditakrifkan melalui literatur. Bahan penggredan

yang tidak linear telah digunakan untuk beberapa reka bentuk lapisan penebat. Model

yang dicadangkan telah digunakan untuk melindungi kawasan yang menghasilkan

medan elektrik tinggi pada penebat kabel skrin. Sifat sesuatu bahan memainkan

peranan penting dalam menyekat medan elektrik merebak di seluruh kawasan

penamatan kabel. Berdasarkan titik pensuisan kira-kira 1kV/cm sifat bahan akan

berubah kepada pengaliran yang berlawanan dengan ketara. Dengan cara ini, ia

menghalang terhasilnya medan elektrik yang tidak diperlukan di atas titik pensuisan

ini. Ketebalan penebat yang berbeza juga telah diperkenalkan untuk mendapatkan

reka bentuk yang lebih baik dan ekonomi bagi menjimatkan sebarang kelebihan kos.

Kajian ini membuktikan keberkesanan untuk kedua-dua kaedah dalam mengawal

tekanan medan elektrik di skrin kabel penamatan telah berjaya.

vii

CONTENTS

TITLE i

DECLARATION ii

DECICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS AND ABBREVIATION xiv

1.0 CHAPTER 1: INTRODUCTION

1.1 Introduction 1

1.2 Effect of Termination on Power Cable Insulation 3

1.3 Problem Statement 3

1.4 Objectives 4

1.5 Scope Project 4

1.6 Project Outline 5

viii

2.0 CHAPTER 2: LITERATURE REVIEW

2.1 Introduction 6

2.2 Power Cable 7

2.2.1 Extruded Polymeric Cable 9

2.2.2 Impregnated –Type Cable 10

2.3 Cable Specification 11

2.4 Damage in High Voltage Cable 13

2.4.1 Ageing 13

2.4.2 Improper Termination Cable 14

2.4.3 Defect by Environmental Condition 15

2.5 Insulation 16

2.6 Electric Parameter 17

2.6.1 Electric Field 17

2.6.2 Electric Stress 18

2.7 Zinc Oxide Microvaristor 19

2.8 Corona Effect at Termination Layer 21

3.0 CHAPTER 3: METHODOLOGY

3.1 Introduction 23

3.2 Finite Element Method (FEM) 25

3.3 Simulation (COMSOL Multiphysics) Flow Chart 27

3.3.1 Geometric Cable Modelling 28

3.3.2 Geometric Parameters 28

3.3.3 Boundary Conditions 30

3.3.4 Generate Mesh 31

3.4 ZnO Microvaristor Modeling 32

ix

4.0 CHAPTER 4: SIMULATION RESULT, ANALYSIS AND

DISCUSSION

4.1 Introduction 33

4.2 The Cable Termination Model without Stress Control 34

4.2.1 Electric Field Distribution at Cable Screen Termination 36

4.3 The Cable Termination with Stress Control 37

4.4 Zinc Oxide Microvaristor Material Design 39

4.4.1 Zinc Oxide Microvaristor Design for Termination Cable 40

4.4.1.1 ZnO Microvaristor Design 1 40




4.5 Comparison ZnO Microvaristor Design Layer 45

4.6 The thickness Effect of the ZnO Microvaristor Shape 46

4.7 Comparison Stress Control Geometry ans Zno Microvaristor

Design 48

4.8 Combination Stress Control Geometry and Zno Microvaristor

Design 49

4.9 Improvement Due to Grading Effect 50

5.0 CHAPTER 5: CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion …………………...……………………………………..51

5.2 Future Work Recommendations ……..…………………...………...53

REFERENCES ………………………….………………………………………..54

x

LIST OF TABLES

Table 2.1 Cable Type Specification 11

Table 3.1 Material Parameters 30

Table 4.1 Tangential Electric Field with Different permittivity 39

Table 4.2 Electric Field in the Critical Area of the Cable Termination Model

with Stress Control for Different Thickness of ZnO Microvaristor

Layer 47

Table 4.3 Improvement Due to Grading Effect 50

xi

LIST OF FIGURES

Figure 1.1 HV power cable in an electricity power system transmission 1

Figure 2.1 Classification of Power Cable 8

Figure 2.2 Extruded Polymeric Cable 9

Figure 2.3 Impregnated-Type Cable 10

Figure 2.4 Cable containing many conductors 11

Figure 2.5 Defect of power cable because of Ageing 14

Figure 2.6 Improper Termination excite breakdown occur 15

Figure 2.7 Environmental Defect 15

Figure 2.8 Electric field represented as vectors 17

Figure 2.9 Field lines used to visualize the electric field 18

Figure 2.10 Characteristic for ZnO micrvaristor material 19

Figure 2.11 Equipotential lines at 5 % interval for dry clean insulator 20

Figure 2.12 Tangential Electric Field along Dry-Clean Insulator 21

Figure 2.13 Uncontrolled cable end – potential 21

Figure 2.14 Corona at outer insulation layer 22

Figure 3.1 Flow Chart Project 24

Figure 3.2 Dimension for 11kV medium cable termination 26

Figure 3.3 Flow chart cable modeling in COMSOL 27

Figure 3.4 Geometric Cable Modeling 28

Figure 3.5 Subdomain Material Setting 29

xii

Figure 3.6 Material for each layer cable model 29

Figure 3.7 Boundary conditions label 30

Figure 3.8 Meshing Model of the cable 31

Figure 3.9 Design of ZnO Microvaristor in insulator 32

Figure 4.1 Termination Electric potential surface 34

Figure 4.2 Cable Equipotential Lines 35

Figure 4.3 Electric Field Distribution at Cable Termination 36

Figure 4.4 Cable Termination Model with Stress Control for Different

Relative Permittivity 37

Figure 4.5 The Equipotential with Different Permittivity 38

Figure 4.6 Proposed Microvaristor characteristics with different switching

threshold at E0 (1) 0.5 kV/cm, (2) 1.0 kV/cm and (3) 5.0 kV/cm 39

Figure 4.7 ZnO Microvaristor Design 1 40

Figure 4.8 Tangential E-Field for Design 1 with Different ZnO Switching

Thresholds 41



Thresholds 42



Thresholds 43



Thresholds 45

Figure 4.15 Equipotential Line for Zno Microvaristor Designs 46

Figure 4.16 Tangential E-Field for Zno Microvaristor All Design 46

Figure 4.17 Tangential E-Field for various Thickness of Stress Control (ZnO

Microvaristor) 47

Figure 4.18 Comparison Electric Field Surface for Different Grading 48

xiii

Figure 4.19 Tangential E-Field Comparison for Two Method 49

Figure 4.20 Equipotential Line for Combination Stress Control Geometry

and ZnO Microvaristor Grading 49

Figure 4.21 Tangential E-Field Combination for Two Method 50

Figure 4.22 Comparison of Tangential Electric Field for all method 50

xiv

LIST OF SYMBOLS AND ABBREVIATION

HV - High Voltage

MV - Medium Voltage

LV - Low Voltage

ZnO - Znic Oxide

XLPE - Cross-Linked Polyethylene

FEM - Finite Element Method

2D - Two Dimension

IEC - International Electrical Commission

LDPE - Low Density Polyethylene

HDPE - High Density Polyethylene

PD - Partial Discharge

kV - kilo Volt

LSR - Linked Silicone Rubber

mm - Milimeter

m - Meter

SiR - Silicone Rubber

E - Electric Field

εr - Relative Permittivity

σ - Conductivity

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Power system is become vital nowadays. Purposes of the power system are to

provide energy or electrical power from source (power generation) to the consumers

in a safe and reliable way. Figure 1.1 shows the high voltage (HV) transmission is

one of the most important part of the system because it delivers almost all powers

that generated by power plants. Electric power can be transmitted by overhead power

lines or underground power cables. Underground cables are commonly used in low

voltage and medium voltage solutions.

Figure 1.1: HV power cable in an electricity power system transmission

2

Utilization power cable becomes extremely high due to usage of electric

power. A good characteristic of cable must be considered to ensure there are no

breakdowns occur during power transmission and distribution process. Distribution

of power has been a frequently improving branch ever since the invention of

electricity. High voltage is needed to get high efficiency in distribution of electricity

[1-2]. Higher voltages lead to a smaller current and therefore a smaller conductor

cross section to transfer a certain amount of power as well as can reduce the cost [2].

The development of cable accessories requires basic understanding of the

stress caused by the electric field in different parts of the insulating structures.

Material properties and product design play a key role in the durability of

terminations and joints. As the voltage goes high, even if the losses become

considerably less, the insulation failure becomes critical.

Dielectric plays important parts in electrical system to insulate the potential

charge materials with the earthed object (including human). Failure in dielectric can

cause electrical breakdown or short circuit in which may introduced the risk of

faulty/damage to the equipment as well as causing potential danger to human. In

general, dielectric can be formed of solid like glass, porcelain, or composite polymer

materials, gases such nitrogen and sulphur hexafluoride and liquid such mineral oils.

These dielectric materials mainly used power transformer of high voltage apparatus

such as cable, insulator and etc. [3].

Cable networks strongly depend on the performance of the way of attached

cable accessories. Therefore, to improve the behavior of the electric field in a

medium voltage termination and a joint, Zinc Oxide Microvaristor was introduced

for stress control on high voltage cable in this project. The optimization of these

issues was done using simulation software called COMSOL Multiphysics. Through

this, couple of simulations would be conducted on various designs of power cables as

well as type of material that have been chosen.

3

1.2 Effect of Termination on Power Cable Insulation

Power cables are very importance in power transmission and distribution

systems. Terminations are the basic accessories of the power cables. They are

required to make connections between lines or to an electrical apparatus. The various

aspects are considered while designing the stress control cable terminations because

they must have the same integrity as their associated cables while making the

connection both all indoor and outdoor applications. The parts of termination must

highly prevents with stress cone control because it will cause discharges and lead to

breakdown.

1.3 Problem Statement

Improper installation of HV cable system in the field can generate different

typical defects in cable accessories. It can be stress on the cable and can cause effect

electric field. The insulation of the cable must maintain due to the high-voltage

stress, ozone produced by electric discharges in air. The cable system must prevent

contact of the high-voltage conductor with other objects or persons. Cable

termination must be designed to control the high-voltage stress to prevent breakdown

of the insulation because this circumstances is easy to affect breakdown. If no stress

were applied, discharges could occur and the life termination would be limited

depending on the stress at the end of shield and the discharge resistance of the

primary dielectric [4].

However, the most common defects caused by bad workmanship are wrong

position of stress cone, and pollutants left in cable termination. Wrong position of

stress cone is usually resulted from not following installation instruction, and this can

result in stress cone does not contact well to outer-semiconductor layer, and leaving

void inside cable termination. Left pollutants are usually caused by casually attitude,

irregular edge of out-semiconductor and irregular surface of insulation. These

defects will enhance potential gradient and electric field that lead to breakdown or

partial discharge [5]. This problem can be addressed by designing stress cone shape

and using non-linear grading material such as ZnO Microvaristors.

4

1.4 Objective

The purpose of this project is to investigate the alternative approach to the

existing technique for optimizing field distribution on cable screen termination in

controlling high field stress that will affect the reliability of the power cable

especially at the termination or joint parts. The main objectives for this project are:

To investigate the problem of the environmental issues that related to the

solid insulation for medium cable.

To propose an optomize technique for controlling high electric field

region at cable screen termination.

To evaluate the performance of different material properties in controlling

the distribution of electric field in medium voltage cable terminations.

To examine the effectiveness of non-linear grading material in controlling

electric field stress at the cable termination.

1.5 Scope Project

This research focuses on single core high voltage power cable. High voltage

cables have various type of shielding [6-7]. For this project used cross-linked

polyethylene (XLPE) polymer was used as insulation in high voltage power cables

and ZnO microvaristor as stress control field material. For the whole project, largely

depends on the ageing of cable and search:

Finite Element method (FEM) has been used computational simulation

for analyze electrical field stress at screen cable termination.

This research deals with primary insulation cross-linked polyethylene

(XLPE) material and semi-conducting insulation.

Focused on 11kV potential for copper conductor and asymmetrical two-

dimensional (2D) mode.

In modeling geometric structure, this project assumes cable termination

profile in ideal condition without nearby structures.

5

1.6 Project Outline

This study is divided into five chapters. The introduction and the information

about the purposes of high voltage cable, problem that normally occur during

cable terminations and electric field controlling constitutes Chapter 1.

Chapter 2 outlines the review of power cable, power cable structure, and also

electric parameter. In Chapter 3, the simulation procedure computer analysis

is reviewed and Zinc Oxide Microvaristor modelling. The results and

evaluation of the simulation are explained in Chapter 4. The effect of

different electric field controlling methods include by applying non-linear

material are discussed in this chapter. Finally, in Chapter 5, conclusion of the

study and future work are covered.

6

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter describes the basic theories that have been used to analyze the

data obtained from the design process and simulation. Power cables are divided into

three groups which are low voltage, medium voltage and high voltage. Low voltage

cable solutions are rated from 300/500 V to 600/1000 V, medium voltage from 1.8/3

kV to 20.8/36 kV and cable solutions greater than 20.8/36 kV are called high voltage

[8]. Stress cone of cable can be categorized depending on company design to or

material used that covered the outer semi-conducting layers [9]. These circumstances

will give impact during termination or jointing cable. Improper circumstances intense

partial discharge issue and corona occur surrounding the cable. It is very dangerous

for human usage and failure transmit power in overhead line. It is also might cause

environmental defect, ageing and also electrical stress otherwise increase of cost.

According to the International Electrotechnical Commission (IEC) International

Standard 60270, Section 3.1 published in 2000, the definition of partial discharge is:

“Localized electrical discharge that only partially bridges the insulation between

conductors and which can or cannot occur adjacent to a conductor. Partial

discharge is in general a consequence of local electrical stress concentrations in the

7

insulation or on the surface of the insulation. Generally such discharge appears as

pulses having duration of much less than 1µs” [10-11].

PD in the insulation is the main cause of aging and failure in insulated cables

[3]. The discharges result in “electrical treeing” that leads to deterioration and

breakdown of the insulation and eventually to cable failure.

2.2 Power Cable

Power cable technology evolved since 1880s when the need for power

distribution cables became pressing. Cable is a conductor which is insulated

electrically and protected mechanically for the safe use of it. The main features found

on the cable construction are conductor, insulator and sheath which are protection for

human safety. Cable is the channel used for a system of transmission and distribution

of electrical energy. The residential and industrial loads today have a highest request

towards their growing density. This requires a tough construction, greater service

reliability, increased safety, and better appearance. These difficulties are easily

overcome by the used of underground cables and a trouble-free service is achieved

under a variety of environmental conditions [11].

There are two ways to transmit and distribute the electrical energy which is

by overhead line transmission or by using underground cable. Earlier underground

cables were mainly used in or near densely populated areas and were operated at low

or medium voltages only, but the present day requirements seek to use them even at

extra high voltages for longer distance [11]. Advantage of using overhead line

transmission, it support a high voltage with the longer distance. Increased working

voltages of the overhead lines require the cables to be insulated for such voltages in

order to meet the requirements of the overhead lines.

8

A power cable consists of the three main components, namely, conductor,

dielectric and sheath [11]. The conductor provides the conducting path for the

current. The insulation or dielectric withstands the service voltage, and isolates the

conductor with the object. The sheath does not allow the moisture to enter, and

protects the cable from all external influences like chemical or electrochemical

attack, fire, etc. Figure 2.1 illustrate classification of power cable which is consists of

insulation of cable, conductor commonly used and sheath for covered the outer semi-

conducting layers.

Figure 2.1: Classification of Power Cable

9

2.2.1 Extruded Polymeric Cables

The development of high voltage such as LDPE, HDPE or XLPE cable

systems goes back to the 1960‟s. Since technology have improved significantly,

providing reliable and maintenance-free products to the utility industry production

and material [12]. Cross-linked polyethylene (XLPE) is the one of the major types of

extruded dielectric insulation. XLPE has high insulation resistance and low dielectric

constant. XLPE has emerged as the generally preferred insulate and in the voltage

range 60-187 kV [13]. It has advantage of a higher permissible operating temperature

of 90º.Nevertheless, the first commercial length of 275 kV XLPE cable was installed

in Japan.

Modern XLPE cables consist of a solid cable core, a metallic sheath and a

non-metallic/polymer outer covering. The cable core consists of the conductor,

wrapped with semiconducting tapes, the inner semiconducting layer, the solid main

insulation and the outer semiconducting layer. These three insulation layers are

extruded in one process. The conductor of high voltage cables can be made of copper

or aluminum and is either round ended of single wires or additionally segmented in

order to reduce the current losses [12].

Figure 2.2: Extruded Polymeric Cable

10

2.2.2 Impregnated-Type Cable

This type of cable is divide into three which is mass-impregnated paper-

insulated cables (solid type), fluid-pressurized cable type, and gas-pressurized cable

type. All this type has own characteristic to transmit the electrical energy. Generally,

for the lapped cable type are conductors, conductor screen, cellulose paper

insulation, insulation screen, impregnating compound, metallic sheath to achieve a

water-impervious barrier [14]. The outer finish might be an extruded plastic jacket.

Impregnated paper became the most common form of insulation for cables

used for bulk transmission and distribution of electric power, particularly for

operating voltages of 12.5kV and above. Paper insulated cables were improved

considerably with introduction of the shielded design of multiple conductor cables by

Martin Hochstadter in 1914. From that, the cable is known as Type H [14-15]. The

cable consisted of two concentric conductors insulated with wide strips of paper

applied helically around the conductor and saturated with rosin-based oil as shown in

figure 2.3 below [16].

Figure 2.3: Impregnated-Type Cable

11

Figure 2.4 shows each is insulated from the others by silk and cotton thread.

Because the insulation in this type of cable is not subjected to high voltage, ,

meaning to say for low voltage the use of thin layers of silk and cotton is satisfactory

an contain many small conductors.

Figure 2.4: Cable containing many conductors

2.3 Cable Specification

Table 2.1 shows some sort of specifications or details for cable includes of

conductor, conductor shield, insulation, insulation shield, and etc. Designing

geometric of cable can be followed by this study of specifications

Table 2.1: Cable Type Specification [15]

Description 11kV

Conductor

Plain stranded annealed Copper wires c/w

longitudinal water blocking features. Copper to have

conductivity of 100%. Minimum cross-sectional area

300 mmsq.

Conductor shield

Shall consist of an extruded semi-conducting

thermosetting compound (non-metallic) applied over

the conductor.

Minimum thickness <0.8mm

DC volume resistivity <100000Ωcm

Maximum void size at the insulation interface

<0.005mm.

12

Insulation

High quality, dry-cured, heat, moisture, ozone and

corona-resistance, XLPE. Suitable for operation in

wet n dry locations.

Dielectric constant at 90º C < 2.5

Power factor of insulation at 90º C < 0.1%

Nominal thickness > 20mm

Minimum thickness > 18mm

Maximum void size <0.05mm

Maximum contaminant size <0.125mm

Insulation shield

Shall consist of a semi conducting XLPE compound

simultaneously extruded onto the insulation. It shall

be firmly bonded to the insulation.

Minimum thickness >1.0mm

DC volume resistivity < 50,000 Ω-cm at maximum

conductor temperature at normal operation.

Maximum void size at the insulation interface <

0.05mm

Maximum protrusion at the interfaces <0.125mm

Extrusion process

The conductor shield, insulation and insulation shield

shall be extruded by simultaneously triple extrusion

process.

Swelling tape

A semi-conducting non-woven or woven, non-

biodegradable (synthetic) swelling tape is to be

applied over and under the metallic screen for

continuous longitudinal watertight barrier throughout

the cable length.

Metallic screen

Metallic screen shall be in the form of concentric

round wire shield consisting uncoated copper wires

helically applied over the swelling tape.

It shall be sized to carry the rated fault current.

13

Radial water

barrier

A radial water barrier consisting of laminated

aluminum tape (PE/AL/PE) is to be incorporated

under the PE over sheath.

Outer protective

covering

The oversheath shall be extruded black PE (ST7)

supplied over the laminated aluminum tape.

The oversheath shall be treated with non-hazardous

and effective termite repellent.

Nominal thickness <4.0 mm

Smallest thickness at any point >85% of nominal

thickness.

Each last section of cable terminating into GIS

switchgear, transformer or any other cable box inside

or outside a substation building and where the cables

are installed on the bridge, the cable shall have a fire-

retardant serving complying to IEC 60332 part 3.

Over conducting

layer

The outer surface of the oversheath shall have a baked

on graphite conducting layer to serve as an electrode

for the voltage test on the oversheath.

2.4 Damage in High Voltage Cable

2.4.1 Ageing

The occurrence of discharges within insulation cavities and the generation of

an electrical tree that highest in cable breakdown is the one off aging process in

polymeric insulated power cables [17]. It is assumed that at the electrical field

exceeding a threshold value, the space charge formed by the PD progresses within

the insulation via channel emanating from the cavity. Penetration depth x is

determined by the electrical field at the tip of the channel. This process is argued to

produce the well-known tree-like structure.

14

Figure 2.5 illustrate defect of power cable because of ageing problem upon

frequently electric discharge occur surrounding insulation layer.

2.4.2 Improper termination Cable

The purpose of a cable termination is to connect an insulated cable to a

circuit. The termination also protects the cable mechanically, keeps moisture out

form the cable and helps to keep the oil inside an oil impregnated paper cable. The

structure of a cable termination depends on cable type, used voltage and installation

environment. For example terminations used outside must be mechanically strong

and completely waterproof. If termination process done without follows the rule will

give dangerous impact to customer and damage the HV equipment and in the while

increase the cost for maintenance fee [18].

The cable termination must withstand the same electric stress that the cable

does. It is important to note that the electric field rises remarkably at the end of the

screening and some sort of grading is required. There are different techniques for

implementing the grading. Geometric grading can be implemented by adding a cone-

shaped (stress cone) insulation on top of the cable insulation and the end of the

screening. Another solution is to add a shaped layer of insulation with a different

permittivity than the surrounding insulation on top of the cutting point of the

screening [18].

Figure 2.5: Defect of power cable because of Ageing

15

Figure 2.6 shows breakthrough could also be an effect of quality issues in the

stress control mass or even in the cable insulation. Wrong electric properties in the

mass could lead to a much higher electric field at the cutting point of the screening.

Improper structure of the mass could also lead to a breakthrough if air bubbles were

left inside the termination. Partial discharges generated inside the air bubbles would

start to slowly burn the insulation and could lead to failure.

2.4.3 Defect by Environmental Condition

Defect accidently cause by the oxidation process. When the cable is exposed to

moisture and oxygen, it will result in rust. This case may lead the cable cannot work

properly, such as shown in figure 2.7 below:

(a) (b)

Figure 2.6: Improper Termination excite breakdown occur

Figure 2.7: Environmental Defect

16

2.5 Insulation

The vast majority of conventional power cables are insulated with either solid

extruded dielectrics or liquid-impregnated papers. The use of the former now

generally dominates the distribution power cable field, whereas the latter is still

extensively used in high-voltage power transmission cable. Although the solid

extruded insulating materials have undergone marked changes with time, the changes

in the liquid-impregnated papers have been substantially not too vital [19]. Insulating

materials are used in electric devices to keep current from flowing where it is not

desired. Generally, the thicker the insulator, the higher the voltage can sustain. In

recent years natural rubber has been completely replaced by synthetic rubbers and

plastics as cable insulation [20].

Many types of XLPE cables of the first generations are suffering from a

degradation problem known as water treeing. This will gradually decrease the

voltage withstand of the cable and eventually a breakdown will occur. The dielectric

compounds as insulates for power cable should possess the several main properties,

for instant:

High insulation resistance

High dielectric strength

Good mechanical properties

Preferably non-hygroscopic, but if hygroscopic it should be provided with an

economical water-tight covering or sheath.

Capable of being operated at high temperatures

Low thermal resistance

Low power factor.

17

2.6 Electric Parameter

The type of cable to be used will depend upon the working voltage and

service requirements. In general, a cable must fulfill the some necessary requirement

such as electric field, electric stress and capacitance of the cable.

2.6.1 Electric Field

The electric field is defined as the force per unit charge that would be

experienced by a point charge at a given point.

(2.1)

Vectors represented as an electric field which continuous entity. Vectors are

drawn as extended arrows which represent the field at the end of the vector. When

using vectors to represent the field it's good to note that we can't draw them all

because the field still exists at every point in space even if we cannot draw a vector

everywhere as shown in figure 2.8 below.

Figure 2.8: Electric field represented as vectors

18

Figure 2.9 (a) and (b) shows more practical way to visualize electric fields

than using vectors is to use electric field lines. Field lines are continuous lines whose

direction is everywhere the same as that of the electric field. Field lines start at

positive charges or at a negative charge or extend to infinity.

Based on Coulomb's law we can conclude that the field is stronger where the lines

are closer to the charge and weaker when they are farther apart. This allows us to

study the field's relative magnitude and direction from field line pictures [21].

2.6.2 Electric Stress

Normally, cable modeled as a coaxial cylinder system. The electric stress at any point in the

insulation material is given by equation [22].

(2.2)

U = Applied voltage

ri= Conductor radius

ro= Sheath radius

x = radial distance to any point in the insulation.

From equation 2.2, it can be seen that the bigger value of sheath radius, the bigger

value will electric stress produce. This will related to the equation below.

Figure 2.9: Field lines used to visualize the electric field

19

(2.3)

It occurs on the surface of the conductor which is the most probable location

for instantaneous failure. This is more likely to occur if serious insulation defects are

near the conductor surface [22].

(2.4)

The minimum stress occurs at the outer sheath and it becomes a critical factor

if insulation defects are in the vicinity of the outer sheath. The mean stress (i.e. E

min) across the insulation is critical if insulation defects are uniformly located

throughout the bulk of the material.

2.7 Zinc Oxide Microvaristor Material

This material is the crucial material in this project as a material to

control stress in cable accessories. ZnO microvaristor can be found inside

heat-curing silicone (HTV) or liquid addition-cross linked silicone rubber

(LSR). Those polymeric material matrices have an elastic property. The

elasticity is necessary to make easier in assemble cable accessories. Figure

2.10 shows the field conductivity of the ZnO microvaristor material. When

switching point reached at 10kV/cm, the material changes its conductivity

extremely. In this way it prevents the occurrence of electric fields which are

above this switching point.

20

Figure 2.10: Characteristic for ZnO micrvaristor material

This material has been demonstrated by previous researcher. Zinc Oxide

Microvaristor were proposed as grading material along the insulator. The electrical

properties of the microvaristor compounds are characterised by a nonlinear field

dependent conductivity. Figure 2.11 illustrate the proposed design, a short length of

ZnO microvaristor compound layer is applied on th core of the insulator near the

insulator HV and ground terminals. Figure 2.11 (a) shows equipotential line without

non-linear grading and figure 2.11 (b) shows non-linear grading were applied.

Results shows after apllied microvaristor grading material between copper and

silicone insulation where 70% interval line refract away from the concerntrated point

[23].

(a) (b)

Figure 2.11: Equipotential lines at 5 % interval for dry clean insulator.

Figures 2.12 show the corresponding electric field profiles along the leakage

path for the clean and polluted graded insulators. As can be clearly observed, the

field distributions on the insulator equipped with grading material under both surface

conditions are improved. Peak magnitudes in the high field regions, particularly at

the HVend are reduced by nearly 30%.

21

Figure 2.12: Tangential Electric Field along Dry-Clean Insulator

2.8 Corona effect at Termination Layer

Regarding previous study, focused on underground cable accessories used in

medium voltage cable. Systems need a highly reliable stress control system in order

to maintain and control the insulation level which is designed for estimated life

times. Figure 2.13 shows the stress concentration at the end of the screen of medium

voltage cables when no stress control system is used.

Figure2.13: Uncontrolled cable end – potential

22

The term „electrical stress control‟ refers to the cable termination function of

reducing the electrical stress in the area of insulation shield cutback to levels that

preclude electrical breakdown in the cable insulation. Figure 2.14 shows breakdown

occur at the edge of outer insulation layer when termination has been done. This

paper focus on metal Oxide-Matrix stress control system to reduce field stress which

has never been attempted before [6].

Figure 2.14: Corona at outer insulation layer

23

CHAPTER 3

METHODOLOGY

3.1 Introduction

To ensure this project will be succeeding, several characteristic of cable will

be study to familiar with cable issues. This study investigates and evaluates different

types of cable termination models by the finite element analysis (FEM) computer

program. The basic steps in simulation procedure steps are explained in this chapter.

The project must consider the cable material and FEM simulation. To achieve the

outcome, some procedure must be followed. First step is to study about power cable

and its properties. After that, electrical parameter of insulating and conductor also

must be studied. There are several ways in gathering the information such as using

references from the books and the most important references are referred to related

journal, previous study, proceedings paper and conference papers.

The simulations with FEM software computer program are demonstrated in three

basic steps which is preprocessing, solution procedure and postprocessing. Which

mean, the preprocessing step involves the model properties (geometrical,electrical)

input and meshing. In the solution procedure, the boundary each geometric

conditions are applied and the program solves the potential values of the model. The

graph plotting and reading of the output is done in the postprocessor step to evaluate

the results.

24

The choice of the appropriate parameters is very important to verify the accuracy of

the output in the analysis. Otherwise, incorrect results and even errors are obtained in

the end of the analysis.

Figure 3.1 shows the flow chart for the whole process for this project of investigation

Zinc Oxide Microvaristor for Stress Control on Medium Voltage Cable

Terminations.

Figure 3.1: Flow Chart Project

Start

Find information regarding of the Power Cable

Cable Conductor Insulator Zno Microvaristor Charateristic

Single phase Three phase

Overhead transmission line & underground

Copper AluminiumLaminated / Lapped

(paper)Non-Linear

gradingExtruded(polymer)

Literature review

Simulation Softwarre (Comsol Multyphysics4.2)

Design Geometric

Analyze of Data

Final Report

Plot graph

Drawing design

Obtain electric field, conductivity, resistivity

Import into Microsoft excell

55

REFERENCES

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Engineering, December 2007.

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118-5

[3] Jasmine James, S.V.Kulkarni, B.R.Parekh , “Partial discharge in High

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International

Conference On Properties And Application of Dielectric Materials, pp. 445-

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http://www.comsol.fi/products/multiphysics

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